Stiffness dampers of various design and size are known. Stiffness dampers relevant to the invention are generally designed and used to damp movement. One such type of movement is the movement of structures during an earthquake.
Throughout history earthquakes have had a devastating impact on society, often resulting in significant economic losses and loss of life. According to the CATDAT Damaging Earthquakes Database, earthquakes caused global economic losses exceeding $500 billion as well as an estimated 20,000 fatalities. The Federal Emergency Management Agency (FEMA) estimates that the minimum average cost of earthquakes to the United States is $5 billion per year. However, a single large earthquake may cost far more than the average annual estimate. For example, the 1994 Northridge, Calif. earthquake alone caused as much as $26 billion, and it is predicted that another large earthquake along the San Andreas Fault in southern California could result in 1,800 fatalities and more than $200 billion in losses.
One way to reduce losses caused by earthquakes is to minimize the vulnerability of civil infrastructure. This can be achieved through infrastructure strengthening and/or the implementation of structural control.
Structural control is of interest here. Structural control can be broadly characterized as active, passive, or semi-active, depending on the hardware requirements. A significant amount of research has been conducted in each category, and widespread application of structural control devices has been achieved. Of the three control types, semi-active control has recently received increased attention due to its adaptability, minimal power requirement, and inherent stability. As a result, several new and innovative semi-active control devices have emerged. One of these, the resetting semi-active stiffness damper (RSASD), has proven effective in reducing the response of structures in the presence of near-field ground motions. This is particularly important, as this type of ground motion is characterized by high peak acceleration and high velocity pulse with long period, and is responsible for the destruction and severe damage to civil infrastructure.
An RSASD generally consists of a piston, a double-acting cylinder filled with compressed air or hydraulic fluid, and a bypass loop with a valve (see FIG. 1a). The cylinder is divided into two chambers by the piston head, and the chambers are connected by the bypass loop. When the bypass valve is closed, the fluid in the cylinder is compressed due to the action of the piston. When the valve is opened, energy stored in the fluid as a result of compression is turned into heat and dissipated. Therefore, for an RSASD installed in a structure, as shown in FIG. 1b, the RSASD adds stiffness to the structure when the valve is closed, and removes stiffness from the structure when the valve is opened.
The ability of a RSASD to add and remove stiffness from a structure correlates to an ability to store and then dissipate mechanical energy. Therefore, the RSASD is capable of extracting mechanical energy from a structure by opening and closing the valve at appropriate time instants. From this basic concept, a resetting mode concept emerged with the aim to maximize the amount of mechanical energy that is dissipated by a RSASD during a given cycle of motion. In the resetting mode, the valve remains closed until drift velocity equals zero, at which time the valve is pulsed open and closed, effectively resetting the stiffness of the device. As a result, a RSASD is always storing mechanical energy from the structure to which it is connected, and only dissipates energy when a maximum amount of energy storage has been reached.
Implementation of the resetting mode concept described above requires the use of feedback components such as encoders for determining piston position, a microcontroller for detecting a change in direction of piston movement, an electric servo-valve for regulating fluid flow, and a small power source for operating these components. One advantage of this resetting mode concept is that it may be implemented based on local information about each RSASD piston position, and does not require knowledge of the structure response at other locations (i.e., it is decentralized control logic).
Another advantage is that the control logic is response dependent, and therefore does not need to rely on accurate information about structural properties which may be estimated incorrectly or change over time. Yet another advantage of the RSASD is the displacement dependent nature of the control force delivered to the structure thereby. This is particularly important for structures subject to near-field earthquakes characterized by high velocity pulses where forces from velocity-dependent devices can often exceed control device capacity, may require excessively large bracing systems for the devices, and can adversely affect the response of the structure. Displacement-dependent control devices such as RSASDs are not susceptible to these effects, and are therefore well-suited for controlling the response of structures subject to near-field motions.
In addition to the aforementioned advantages, structural control systems using RSASDs are simple, reliable, and relatively inexpensive relative to other semi-active control systems. This can be attributed to the construction of the device, which is based on minor external modifications to existing pneumatic or hydraulic damper technology that is well-developed and readily available.
Typical of semi-active control technologies, a RSASD also has several complexities associated with its operation. First, the control law for a RSASD requires that stiffness be removed from an associated structure when it has reached maximum displacement, or zero velocity. This is achieved through a feedback control system consisting of a sensor, microcontroller, and a small actuator to control the valve. As a result, the feedback control system is disproportionately complex relative to the feedback law.
Furthermore, the feedback control system is designed such that the valve is pulsed open and closed when the piston has reached its maximum displacement, i.e., when there is a change in sign of the piston velocity. However, this means that any noise (interference) in the sensor signal, or any high frequency small amplitude structural vibrations, could also trigger the valve, thereby resetting the device at the wrong time. To prevent this, a deadband and a threshold on the position signal must be used. The threshold is used to ensure that a predetermined minimum piston displacement has occurred before resetting the device. The deadband eliminates resetting of the device based on localized peaks in the position signal that do not correspond with the maximum position of the piston.
As a result of the threshold, resetting only occurs after the piston has moved some minimum distance. As a result of the deadband, the valve is triggered a short time after the actual maximum displacement of the piston has occurred.
It can be understood from the foregoing commentary that, while RSASDs have advantages when used for structural control, there is nonetheless a need for a simpler device that provides similar results. Embodiments of the invention satisfy this need.